Good topic de facto; this is one of those things that has bothered me as well.
As was discussed by Mad Flt Scientist in this thread
http://www.pprune.org/tech-log/42082...aircrafts.html, I think 400'AGL is a certification altitude. I have searched and have never been able to find any reference to 400'AGL anywhere else.
Perhaps Boeing advises 400' OEI vs 1000'AGL for normal ops because the aircraft will not meet the OEI performance requirements if flaps are carried to 1000'AGL?
I used to fly the Gulfstream 2 and 3. On the 2, OEI SOP had us start cleaning things up at or above 400'AGL. However, in the 3 we carried takeoff configuration to 1500'AGL OEI; we were told this was because the 3's OEI performance exceeded the minimum requirements of Part 25. Gulfstream's 1500'AGL OEI profile continues in the 4 and 5.
Unfortunately, Gulfstream has recommended for years that for normal takeoffs the profile is gear up, flaps up.
I refuse.
Best,
GC